Metro XFI Cam Advance Experiment


Well, I just finished with a fairly long experiment. I have been moving the cam timing around on my car to see what effect it has on mileage. Keep in mind that I have shaved the head .040" so the car cannot tolerate as much advance as when it was stock. I have tried from -2 to +11, and for each of them I ran the maximum ignition timing that I could run without knock. I gave up trying to do 2 tanks each due to the weather, and my having to drive less to work means that it takes me 2-3 weeks to run a tank of gas out now so it would be almost a year to do this test the long way. I did short tanks of at least 100 miles and filled the tank to the top with the car sitting at an angle to keep the air bubbles out.

The numbers are probably not 100% perfect, but they are pretty close and fit a curve reasonably well, so they still give useful information.

Cam Position VS MPG

The curve is not perfect; I am pretty sure the 5 degree advance number is a bit lower than what it should have been. I would say advancing a stock XFI engine 5 degrees is ideal, and advancing one with the head shaved 3-4 degrees is probably ideal. I will redo this experiment later on going in one degree increments from 2-5 degrees advanced to see if I can fine tune my timing. I drove the car for a full tank of gas with the cam at 11 degrees advanced--mainly because of the bad weather I did not feel like changing it back. The car accelerates erratically and surges badly at 11 degrees. Also it knocks very easily, so the best I could do was to run the ignition at 3 degrees advanced without knock. I got 49 mpg on that tank, and even though the number is higher than the number on my test run, I still do not think updating the chart will help. It is pretty obvious where the peak of the curve will fall if the chart was smoothed out, so anything over 5 degrees advanced is a waste of time on the XFI camshaft.

With the car at -2 degrees, I could really crank up the ignition timing, so that probably helped the mileage. The car ran great and was more than happy to spin the tires when pulling out if I was not careful with the clutch and throttle. The acceleration was really good, and the top end seemed to keep on going. I think the car with the gears it has in it now would probably do really well at higher speeds only at 2 degrees retarded, but the bottom end of the car I could tell had less power, and it did not like at all being in gear under 1500 RPM. It would buck and jerk pretty badly. It kicked so hard that it pulled the ignition wire out of the coil once while driving it slowly, killing the motor. I had to pull over to the side of the road and put it back on. At 2000 RPM and higher, it felt great though. After driving it with the cam like that, I got an idea for another experiment if I ever have time--to compare the cam position and the rpm in 5th gear to find where it gets peak mileage. I bet if I did that, the peak would be at around 2700 RPM for -2 and under 2000 for +5 degrees advanced. I am not sure what those peak MPG numbers would be, but it would be an interesting experiment.

The car did not feel much different in the 0 to 5 degrees advanced range. It picks up a bit more bottom end grunt at around 5, and I can drive 20 mph in 5th gear and still accelerate without trouble. The top end is slightly less than stock, but if you are trying to get better mileage, you are not supposed to be running over 4000 RPM anyway. It drives smooth and has good power going up hills without downshifting. I really think if the curve was calculated perfectly, it would be pretty flat from 2-5. After doing the test in 1 degree increments, I should be able to show that it really isn't a peak. I like the way the car drives with the cam in this range, so I think that from the numbers I have so far that 3 degrees advanced seems ideal even though it can probably be 2-4 degrees and not make a difference.

I will try and do this test when the weather finally breaks and I don't have to work on the car in the cold and snow. I like the way the car feels at 3 degrees advanced, so for now the car will stay at 3 degrees. I can run 6 degrees of ignition advance, so it seems pretty good all around and should get me the best mileage possible. For anyone interested in changing their cam timing, it is pretty easy to do, but keep in mind my car has higher compression than stock, so it cannot tolerate the cam being advanced as much as a stock car. If you are running the stock head, I would add probably 2-3 degrees to my numbers.